The enteric nervous system is a unique region of the peripheral nervous system that is particularly well-suited for studies of the development of neuronal diversity. It has a structure and chemistry that resembles the CNS but is a much less complex system. The enteric nervous system has a limited number of known types of neuron (including intrinsic cholinergic neurons, serotonergic neurons and neurons that store peptides as well as the axons of extrinsic adrenergic neurons) and their transmitter mechanisms provide an array of convenient markers for neuronal detection. Preliminary studies have indicated that a sequential order is followed in the appearance of enteric neurons that is reproduced in several mammalian species and also in birds. This observation, and the recent evidence that the microenvironment of the tissue in which development occurs is vital to the final determination of such neuronal characteristics as transmitter choice, has led us to want to test the hypothesis that the microenvironment of the gut is critical in enteric neuronal development and to the deveopment of diversity among enteric neurons. Normal development of the enteric nervous system will first be examined, utilizing transmitter-specific markers in order to obtain clearly defined quantitative end-points against which the effects of experimental procedures can be judged. These studies will also use organotypic tissue culture techniques and quantitative electron microscopy. After normal development has been examined, manipulations of the enteric microenvironment will be attempted.